1. Field of the Invention
The present invention relates to the manufacturing and testing of integrated circuits. More particularly, the present invention relates to electrical contacts on probe elements such as on membrane probe cards used in integrated circuit testing.
2. Description of Related Art
Integrated circuit chips are formed using fabrication processes which build many chips on wafers, which are for example 6 or 8 inches in diameter or larger. While the chips are still on the wafer in as-formed positions, a membrane probe can be used to electrically sense the performance of the circuits on the chips. See for example U.S. Pat. No. 5,180,977 entitled MEMBRANE PROBE CONTACT BUMP COMPLIANCY SYSTEM invented by Huff; U.S. Pat. No. 4,980,673 entitled FORCE DELIVERY SYSTEM FOR IMPROVED PRECISION MEMBRANE PROBE invented by Huff et al.; U.S. Pat. No. 4,918,383 entitled MEMBRANE PROBE WITH AUTOMATIC CONTACT SCRUB ACTION invented by Huff et al; and U.S. Pat. No. 4,906,920 entitled SELF-LEVELING MEMBRANE PROBE invented by Huff et al. In accordance with this technology, costs savings are possible through early identification of faulty circuits before they are separated from the wafer, and thereafter assembled into chip packages.
Good electrical contact between the probe contacts and chip contacts, often called pads, is essential for accurate evaluation. The contacts of chips on the wafer are formed typically of aluminum or combinations of aluminum with other metals, and have a natural skin of hard aluminum oxide, or other natural hard skin caused by oxidation or other reactions with the environment. Wear of the probe contacts occurs as a result of repeated contact with the hard aluminum oxide skin, resulting in a degradation of the accuracy of the measurements over time.
In the prior art, the probe elements are formed with a contact bump technology usually based on nickel sulfamate plating to build up bump contacts. However the nickel deposit as a result of this plating technique is too soft for extended use, for example beyond about fifty thousand to one hundred thousand touchdowns. With such extended use, the contour of the nickel bump frequently develops an undesirable flattened area.
One prior art technique for improving the performance of the contact bumps involves creating a rough textured surface on the bump. See U.S. Pat. No. 5,487,999 entitled METHOD FOR FABRICATING A PENETRATION LIMITED CONTACT HAVING A ROUGH TEXTURED SURFACE invented by Farnworth. However, the roughening of the surface as described in the Farnworth patent does not overcome the problems of the softness of the nickel bump, while it may improve the ability to penetrate the hard skin on the contact pad.
Also, examination of the tips of worn nickel bump contacts at high magnification (scanning electron microscope) indicates that the soft nickel contact surface tends to accumulate aluminum oxide. The embedded aluminum oxide ultimately degrades performance by increasing the bump to pad contact resistance. Increased friction and mechanical resistance to sliding are also associated with contact wear and aluminum oxide pickup.
Accordingly, there is a need for a technique for extending the lifetime of the contact bumps used in membrane probe cards and in other testing situations where the contacts are required to make many thousands of contacts over their useful life.
The present invention provides an improved wear resistant bump contact produced by the inclusion of small particles of hard materials in the conductive material of the contact bump, preferably by co-deposition at the time of electroplating of the bump bulk material. Desirable attributes of the small particles of hard material include small particle size, hardness greater than the hardness of the bulk material of the contact bump, compatibility with the plating conditions, and electrical conductivity. Nitrides, borides, suicides, and carbides are typical hard interstitial compounds suitable for use with metals like nickel, and which satisfy these desirable attributes.
The present invention can be characterized as a contact probe comprising a probe substrate having a set of conductive traces on the substrate. An array of contact bumps is formed on the substrate by which electrical contact is made to the set of conductive traces. The contact bumps have a raised portion extending away from the substrate and a contact surface on the raised portion. The contact bumps comprise conductive material and include particles in the conductive material at or near the contact surface. The particles comprise material harder than the conductive material to improve resistance to abrasion and to provide improved contact during the probing operation. The conductive material according to a preferred aspect of the invention comprises a metal and the particles comprise a hard conductive compound. In another aspect of the invention, a thin layer of noble, non-oxidizing metal is formed on the contact surface to improve resistance to spark erosion and other contact characteristics.
In one preferred example, nickel bulk material and silicon carbide particles are utilized. Silicon carbide is commercially available in sizes suitable for co-deposition and produces a wear resistant composite electrodeposit when used in combination with nickel and other metals. To achieve the composite electrodeposition of silicon carbide in a nickel matrix, silicon carbide of a particle size significantly smaller in the dimension of the bump is mixed with the plating solution. The carbide particles become imbedded throughout the nickel matrix as the nickel grows by electrodeposition. The co-deposited particles lodged at or near the plated surface provide the surface with hard edges to cut through the aluminum oxide skin of the contact pads on the silicon wafer. The nickel bumps thus have better opportunity to make electrical contact with aluminum metal in a scraped and cleared path on the pad. The silicon carbide is also electrically conductive, and therefore does not interfere with electrical measurements made through the contact bumps.
In addition to silicon carbide, other binary or tertiary hard interstitial compound materials of small particle size suitable for inclusion alone or as admixtures by co-deposition in nickel and other metals useful for contact bumps include titanium nitride, zirconium nitride, boron carbide, tungsten carbide, chromium carbide, and other nitrides, carbides, silicides and borides. Other hard materials such as diamond particles may also be co-deposited either alone or as an admixture to provide wear resistance to the contacts.
In one variation, the entire bump volume is comprised of metal/particle composite deposit. In another variation, substantially most of the bump may be formed by plating metal only with a subsequent cap of the metal/particle co-deposit being thereafter deposited to form a cap on the metal. In yet another alternative approach, the metal/particle co-deposition is conducted during a first part of the process under conditions which cause incorporation of a low number of particles, and during a second part of the process under conditions which cause a higher concentration of particles to be co-deposited at or near the surface of the bump.
In yet another variation of the invention, the bump of metal/particle co-deposited material is further coated by a thin cap layer of noble, non-oxidizing metal to prevent electrical erosion by arcing as contact is made and broken from the pad. Rhodium and ruthenium are suitable metals and can be electrodeposited over the composite bump structure.
Accordingly, a new composite structure, contact bump is provided according to present invention suitable for use with membrane probe cards. Testing the preferred embodiment suggests that wear resistance and resistance to abrasion are greatly improved over prior art systems.
Other aspects and advantages of the present invention can be seen upon review of the figures, the detailed description and the claims which follow.